Advance melt zone production of a monocrystalline rod

ABSTRACT

METHOD OF CRUCIBLE-FREE ZONE MELTING A CRYSTALLINE ROD WHICH COMPRISES SUCCESSIVELY PASSING ALONG THE ROD IN THE AXIAL DIRECTION THEREOF AN ADVANCE MELTING ZONE FORMED IN THE ROD TO A DEPTH SMALLER THAN THE RADIAL THICKNESS OF THE ROD AND A SUBSEQUENT MELTING ZONE FORMED IN THE ROD AND EXTENDING ACROSS THE ENTIRE CROSS SECTION OF THE ROD.

June 15, 1971 w. KELLER ETAL 3,535,008

ADVANCE MELT ZQNE PRODUCTION OF A MONACRYSTALLINE ROD Filed Sept. 22,1967 19 L i I 17 17 SOLID-MELT x INTERFACE 41o 7cm. 4107 cm. SOLlD-MELTINTERFACE I wa /Jul United States Patent US. Cl. 23301 9 Claims ABSTRACTOF THE DISCLOSURE Method of crucible-free zone melting a crystalline rodwhich comprises successively passing along the rod in the axialdirection thereof an advance melting zone formed in the rod to a depthsmaller than the radial thickness of the rod and a subsequent meltingzone formed in the rod and extending across the entire cross section ofthe rod.

Our invention relates to method of crucible-free zone melting acrystalline rod, especially of semiconductor material.

It is known to transform polycrystalline rods into rodshapedmonocrystals by fusing a monocrystalline seed crystal to one end of apolycrystalline rod, and to pass a melting zone repeatedly through thecrystalline rod starting from the fused junction of the seed crystaltherewith. This zone-melting process is frequently carried out without acrucible, i.e. a melting zone produced with the aid of a high frequencyheating coil surrounding the rod is passed through the crystalline rod,which is vertically disposed between a pair of rod end-holders. The endholders can be rotated in the same or opposite rotary directions.

To save time and energy, an effort is usually made to transform thepolycrystalline rod to a monocrystal by at most a single melting zonepass through the rod. Difliculties are sometimes encountered therebybecause entire granules can become loosened at the edge of the boundarysurface or interface between the polycrystalline rod and the meltingzone when the polycrystalline material is being melted, and thoseloosened granules can then find their way without melting to theboundary surface between the melting zone and the rod portionsolidifying or recrystallizing from the melting zone and can causedisturbance of the monocrystal formation at that location. An additionalmelting zone pass through the rod is then necessary in order to obtaincomplete formation of the monocrystal.

It is accordingly an object of our invention to provide method ofcrucible-free zone melting a crystalline rod which avoids the foregoingdisadvantage of previously known methods of this general type and which,more particularly, a-voids disturbance of monocrystal formation bynon-melted particles.

With the foregoing and other objects in view, we accordingly providemethod of crucible-free zone melting a crystalline rod which comprisessuccessively passing along the rod in the axial direction thereof anadvance melting zone formed in the rod to a depth smaller than theradial thickness of the rod and a subsequent melting zone formed in therod and extending across the entire cross section of the rod.

In accordance with further features of our invention, the advancemelting zone is annular in shape and has a depth of from /s to /3, andpreferably /2, the radius of the rod. In one mode of the method of ourinvention, both melting zones are passed together through the rod, theadvance melting zone travelling in advance of the subsequent meltingzone spaced at a predetermined distance Patented June 15, 1971therefrom. It is advantageous when the melting zones are mutually spacedapart a distance of from 4 to 7 cm.

In another mode of the method of our invention, an advance melting zoneis first passed through the entire rod and, thereafter, the subsequentmelting zone is passed through the rod.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as method ofcrucible-free zone melting a crystalline rod, it is not intended to belimited to the details shown, since various modifications and changesmay be made therein without departing from the spirit of the inventionand within the scope and range of equivalence of the claims.

The method of the invention, together with additional objects andadvantages thereof, will be best understood from the followingdescription when read in connection with the accompanying drawings, inwhich:

FIGS. 1 and 2 are longitudinal views of a crystalline rod and twoembodiments of a device for crucible-free zone melting the rod inaccordance with our invention; and

FIG. 3 is a perspective 'view of a component of the device of FIG. 2.

Referring now to the drawings and first, particularly, to FIG. 1thereof, there is shown a crystalline rod 10, for example of silicon,vertically supported in a device for crucible free zone melting the rod10. At the upper end of the rod 10, as viewed in FIG. 1, a carbon rod 11is fused thereto and is secured in a holder 12. At the lower end of therod 10, as viewed in FIG. 1, a narrow monocrystalline seed crystal 13 isfused thereto and is secured in a holder 14. The rod 10 is surrounded bytwo high frequency heating coils 15 and 16 which are connected to acommon high frequency electrical generator (not shown) or to twodifferent high frequency generators (not shown). The spacing between thecoils 15 and 16 is such that the melting zones produced thereby in therod 10 are located about 4 to 7 cm., and preferably 5 cm., from oneanother.

To carry out the method of the invention, the rod 10 is moved in theaxial direction thereof relatively to the high frequency coils 15 and;16 a distance until the high frequency coil 16 surrounds the carbon rod11. Then the high frequency coil 16 is electrically energized so that anincandescent zone is formed in the carbon rod 11. This incandescent zoneis then passed by relative motion of the colis 15 and 16, on the onehand, and the rod 10, on the other hand, through the rod 10 to thelocation at which the seed crystal 13 is engaged with the rod 10. Assoon as the incandescent zone has reached the location of contactbetween the seed crystal 113 and the rod 10, the electrical energysupplied to the coil 16 is increased so that the rod material melts andan annular advance melting zone 17 is formed below the surface of therod. By relative motion between the rod 10 and the coils 15 and 16, theannular advance melting zone 17 is passed through the rod in a directionfrom the lower to the upper end thereof, as viewed in FIG. 1. The depthof the annular advance melting zone 17 is from .4-, to /3 the radialthickness of the rod 10, a depth of /2 the rod radius being mostfavorable. When the coil 15 has reached the location of engagementbetween the seed crystal 13 and the rod 10, the relative motion betweenthe rod 10, on the one hand, and the coils 15 and 16, on the other hand,is interrupted, and electrical energy is supplied to the coil 15. Afterthe seed crystal 13 has been fused to the roc '"10, the relative motionis continued in the same direction and the subsequent melting zone .18,produced by means of the coil 15 and extending over the entire crosssection of the rod 10, is passed together with the annular advancemelting zone through the rod 10. The formation of the monocrystal isaccordingly free of trouble, because the granular polycrystallinestructure, which might otherwise cause disturbances, is homogenized bythe advance melting zone below the surface of the rod.

The high frequency coil 16 producing the annular advance melting zone 17is de-energized shortly before it reaches the carbon rod 11.

A wire loop 19, is shown in partial sectional view in FIG. and inperspective in FIG. 3, also energized with high frequency current butnot surrounding the crystalline rod 10, can also be used for producingan advance melting zone in the rod $10. The wire loop 19 is locatedalongside the semiconductor rod 10, for example in a curved planeparallel to the surface of the rod 10. The rod holder 12 and the upperpolycrystalline rod portion held thereby, as shown in FIG. 2, must thenbe rotated so that the entire peripheral surface of the rod portion atthe particular level of the wire loop 19 sweeps past the loop and isheated thereby. The relative motion of the rod 10, on the one hand, andthe high frequency coil 15 as well as the wire loop 19, on the otherhand, is effected in such a way that the material of the rod firstpasses the wire loop 19. In other words, for example, as viewed in FIG.2, the rod 10 can either be moved upwardly while the loop 19 and coilare held stationary, or the rod 10 can be held stationary while the loop'19 and the coil 15 are moved upwardly in succession along the rod 10.

If the monocrystalline seed crystal in the devices of FIGS. 1 and 2 wereclamped in the upper rod holder i.e. the monocrystalline re-solidifyingrod were being pulled downwardly from above, then the high frequencycoil 16 or the wire loop 19 producing the advance melting zone must bedisposed below the high frequency coil 15 producing the subsequentmelting zone.

It is advantageous to energize the high frequency coils 15 and 16 andthe wire loop 19 with two different high frequency generators. Thus,neither the supply of electrical energy to the high frequency coil 15nor the supply of electrical energy to the high frequency coil 16 or thewire loop 19 can affect one another.

In a further modification of the invention, only a single high frequencycoil 15 can be used in the device of FIG. 1. In such a case, thecrystalline rod :10 and the high frequency coil 15 are initiallydisplaced in the axial direction relative to one another for a distanceuntil the high frequency coil 15 surrounds the carbon rod 11. The highfrequency coil 15 is then energized, and an incandescent zone is formedin the carbon rod 11. The incandescent zone is then passed into the rodmaterial adjoining the carbon rod 11 by relative motion between the highfrequency coil 15 and the crystalline rod 10 in the axial direction ofthe rod 10. Thereafter, the supply of electrical energy to the highfrequency coil #15 is increased so that an annular advance melting zoneis formed below the surface of the crystalline rod 10. The advancemelting zone is then passed by a further relative motion of the coil 15and the rod 10 in the axial direction thereof through the crystallinerod 10 to the location at which the rod 10 engages the seed crystal. Theseed crystal 13 is then fused by the coil 15 to the rod 10 and,thereafter, the electrical energy supplied to the high frequency coil 15is again increased and a subsequent melting zone extending over theentire cross section of the rod 10 is produced by the coil 15 and ipassed through the rod 10 by relative motion of the rod 10 and the coil15 in the opposite direction.

As aforementioned, the relative movements in the axial direction of therod 10 can be effected either by displacing the high frequency coils 15and 16 and the wire loop 19 while the crystalline rod 10 is heldstationary or by displacing the crystalline rod 10 while maintaining thehigh frequency coils 15 and 16 and the wire loop 19 stationary.

Although not shown in the drawing, the device according to our inventionis installed in an evacuated vessel or in a vessel containing an inertatmosphere such as argon, for example. Auxiliary equipment for carryingout a crucible-free zone melting process in accordance with theinvention are not shown since they are not essential to the method ofthe invention and are, moreover, well known to the man of ordinary skillin the art of zone melting.

We claim:

1. Method of crucible-free zone melting a crystalline rod so as totransform it into a monocrystalline rod, which comprises successivelypassing along the rod in the axial direction thereof from amonocrystalline seed crystal fused to an end of the rod an annularadvance melting zone formed in the rod to a uniform depth smaller thanthe radial thickness of the rod and a subsequent melting zone formed inthe rod and extending across the entire cross section of the rod.

2. Method according to claim 1 wherein the advance melting zone has adepth of /5 to /3 the radial thickness of the crystalline rod.

3. Method according to claim 1, which comprises passing both meltingzones, spaced apart from one another a distance of substantially 4 to 7cm., simultaneously through the rod.

4. Method according to claim 1, which comprises passing both meltingzones simultaneously through the rod.

5. Method according to claim 1, which comprises initially passing theadvance melting zone through the entire rod and thereafter passing thesubsequent melting zone through the rod.

6. Method according to claim 1, which comprises relatively displacing,in the axial direction, the crystalline rod and a pair of coaxiallyspaced high frequency coils surrounding the rod and energized atdifferent intensities to form the respective melting zones therein, sothat the material of the rod first passes the coil energized at lowerintensity.

7. Method according to claim 6, which includes separately energizing thehigh frequency coils from respective generators.

8. Method according to claim 1, which comprises relatively displacing inthe axial direction the crystalline rod, on the one hand, and anenergized high frequency coil surrounding the rod as well as a wire loopaxially spaced from the coil and located adjacent the rod, on the otherhand, the loop being energized at lower intensity than the coil, so thatthe material of the rod first passes the wire loop, and simultaneouslyrotating the part of the m1- terial first passing the wire loop.

9. Method according to claim 1, which comprises relatively displacing inthe axial direction the crystalline rod and a high frequency coilsurrounding the rod and energizable at varying intensity, While the coilis energized at a relatively loW intensity, and thereafter increasingthe intensity of the energy supplied to the coil and relativelydisplacing the rod and coil in the opposite axial direction while thecoil is energized at the increased intensity.

References Cited UNITED STATES PATENTS 2,914,397 11/1959 Sterling -633,023,091 2/1962 Smith 23-301 3,036,898 5/1962 Brock et al 23-3013,092,462 6/1963 Goorissen 23-301 3,177,051 4/1965 Scholte 23-2933,210,165 10/1965 Van Run et al. 23-301 3,258,314 6/1966 Redmond et al23-301 3,423,189 1/1969 Pfann 23-301 S. LEON BASHORE, Primary ExaminerR, T. FOSTER, Assistant Examiner US. Cl. X.R. 23-273

